Footwear and illumination modulate muscle cocontraction strategies and joint work redistribution during stair-to-ground transition

Reduced illumination diminishes visual input, while footwear cushioning alters impact absorption. How these factors affect joint loading and muscle activation during the stair-to-ground transition remains unclear. This study investigated the effects of illumination and footwear on joint kinetics and muscle coordination during the transition step of stair descent, elucidating regulatory mechanisms and potential interactions. Twenty-four healthy adults performed a stair-to-level transition under four conditions (illumination: bright vs. dim; footwear: shoed vs. barefoot). Kinematic trajectories, ground reaction forces, and electromyographic signals were recorded for the dominant side during the transition step's stance phase. Analysis used two-way repeated-measures ANOVA and one-dimensional statistical parametric mapping. Illumination effects primarily manifested as increased hip flexion-extension negative work in bright conditions, whereas dim conditions decreased knee abduction moments and adduction-abduction negative work. The shoed condition increased ground reaction force loading rates and joint moments across multiple degrees of freedom at the hip, knee, and ankle, significantly elevating knee flexion-extension and hip rotational positive work. Conversely, the barefoot condition showed higher midstance ground reaction forces, greater ankle plantarflexion moments, increased hip rotational negative work, and higher knee and ankle muscle coactivation indices. Compared with illumination, footwear exerts a more pronounced effect on lower limb joint moments, negative work, and muscle activation during the stair descent transition phase. These findings provide directions for future research on footwear-ground interaction properties and lower limb motor control.<b>NEW & NOTEWORTHY</b> This study reveals that footwear impacts lower limb biomechanics more significantly than illumination during the stair-to-ground transition. Although dim light alters knee stability and hip work, footwear cushioning fundamentally redistributes joint moments and negative work. Notably, barefoot conditions trigger higher muscle coactivation and ankle loading to manage impact. These findings highlight how environmental and equipment factors interact, offering critical insights for improving footwear design and motor control strategies during high-risk locomotor transitions.